There are various recording media, including a semiconductor recording medium, a magnetic disk, an optical disk, and a magneto-optical disk, which record various types of digital data (hereinafter, referred to as the “content”), such as music content, animation content, and still image content.
Among them, in particular, the semiconductor recording medium has characteristics of being compact and lightweight, and therefore can be used in various devices, including digital cameras, cell phones, mobile music players, television sets, and DVD recorders. Representative examples of the semiconductor recording medium include an SD memory card, Memory Stick®, and Compact Flash®.
Some of such semiconductor recording media have a copyright protection function as the SD memory card does. The copyright protection function prevents copyrighted content from being illegally copied, thereby allowing safe recording and playback.
The following is a description of an exemplary content storing method realized by a semiconductor recording medium having the copyright protection function.
The semiconductor recording medium includes: a protected area that cannot be accessed by a host device (a data playback device or a data recording device) until the semiconductor recording medium and the host device mutually authenticate their validities; and an user data area that can be freely accessed by the host device. Content that has been encrypted (hereinafter, referred to as the “encrypted content”) is recorded to the user data area, and a key used to encrypt the content (hereinafter, referred to as the “content key”) is stored to the protected area.
The content recorded in such a manner can be prohibited from being played back in invalid devices for which mutual validity authentication is not possible. This is true because the invalid devices cannot read the content key and therefore cannot decrypt the encrypted content. Furthermore, although only the encrypted content stored in the user data area can be copied to another recording medium, the invalid devices cannot copy the content key, and therefore no content key can exist in the copy destination recording medium. Thus, it is possible to prohibit illegally-copied encrypted content from being played back.
Conventionally, types of content that are stored to semiconductor recording media having the copyright protection function in accordance with the above-described storing method are mainly music content ripped from music CDs purchased by users, and music and book content downloaded from content providers by paying a given amount of money using personal computers (PCs) and cell phone terminals that are connected to Internet networks. A popular usage pattern is to record such content to the semiconductor recording medium, and play the music content stored in the semiconductor recording medium, or read the content in the case of a book, using a compliant device such as a compact semiconductor player or a cell phone terminal.
Furthermore, in recent years, along with such a content usage pattern, a so-called “superdistribution” method for managing content separately from keys has attracted attention, in which content keys and encrypted content are stored to different recording media, such that the content keys are stored to a semiconductor recording medium having the copyright protection function, whereas the encrypted content is recorded to a magnetic disk, an optical disk, or a magneto-optical disk.
Such a key-separation-type content management method allows the encrypted content to be distributed to users in the form of a magnetic disk, an optical disk, or a magneto-optical disk, which has the content prestored therein and is relatively inexpensive compared to the semiconductor recording medium. Therefore, content providers, which distribute content, can create various business opportunities, while the users can enjoy more convenience than they could do conventionally.
The key-separation-type content management method requires correlation between a content key and encrypted content when the encrypted content is recorded or played back. One method for such correlation is to assign uniquely identifiable ID information to both the content key and the encrypted content, and correlate them using their respective pieces of ID information as indices. In Patent Document 1, the content key and the encrypted content as described above are assigned with their respective pieces of unique ID information, and stored and managed in different locations, and the pieces of ID information are used as indices to decrypt the encrypted content.    Patent Document: Japanese Laid-Open Patent Publication No. 2003-30056